Heretofore, it has been known to utilize flow-through cells for the ionic or pH measurement of aqueous solutions as described in the article, "Theoretical and Practical Aspects of Ion-Selective Electrodes in Continuous-Flow Systems", by John R. Potts, ADVANCES IN AUTOMATED ANALYSIS, Vol. 3, Technicon International Congress, 1976, Mediad, Inc., Tarrytown, N.Y. 10591. This type of test cell lends itself nicely to measurement of the ionic or pH values for solutions because the high flow rate generally provides adequate mixing on the sensing elements, so as to achieve reproducible and accurate measurements. To achieve reproducible and accurate measurements with limited amounts of sample at high processing rates, however, requires that the solution stream be segmented, e.g., by air segments, to wash the cell clean from test sample to test sample, as shown in U.S. Pat. No. 3,840,438, assigned to a common assignee. Even with high flow rates, stagnant layers may exist upon the sensing membranes or elements to affect the accuracy of the measurement. These stagnant layers are effectively eliminated by the air segments in the stream as they virtually occlude the flow passage of the cell.
A problem, however, is associated with the passage of occluding air segments in the sample stream flowing through a test cell. In a pH cell, for example, when one or more air segments fill the space between the pH membrane and the porous liquid junction of the reference electrode of the cell, the effective solution resistance greatly increases due to "impedance shock" and "streaming potentials" arise, which degrade the pH measurement. By "streaming potentials" is meant the voltage difference developed along a portion of a conduit through which liquid is flowed because of the flow pattern of the liquid. The liquid flowing through the conduit has a double charge consisting of a fixed layer of charge on the surfaces of the inner wall of the conduit and a counter charged layer partially distributed in the bulk of the solution. A voltage difference in the direction of liquid flow is thus established, and a reverse flow of ionic charge through the liquid will occur. The magnitude of the streaming potential is inversely proportional to the conductance of the solution; and is a function of conduit geometry. By "impedance shock" is meant the sudden increase in solution resistance caused by the inert segment entering between sensing elements in the conduit. This sudden resistance additionally causes a ringing signal (noise) in the potential. In addition, "surging" within the stream itself, further degrades the measurement by introducing oscillations (noise) into the signal. By "surging" is meant that the inert segments, which may be air, are not evenly spaced due to flow abnormalities associated with the insertion of the air into the stream or due to the pumping action driving the solution stream. As a result, reliable ionic measurements using segmented streams are not possible. The degradation is particularly severe when dealing with medium to poorly buffered solutions, which usually exhibit low conductivities. This is true, because the magnitude of the noise increases as a function of decreasing solution conductivity, while the magnitude of the signal is independent of solution conductivity. Signal sampling and averaging techniques could normally be applied to improve the signal-to-noise ratio, but due to the variations in the flow conditions, these techniques are not entirely satisfactory.
The invention seeks to eliminate the aforementioned problems, by reducing the magnitude of the potential drop and noise in the test cell measurement generated by the continuously flowing, air-segmented, weakly conducting solution stream. This is accomplished by inserting a conductive wire down the center or mid-portion of the flow-through cell, which acts as a low-impedance shunt through the fluid, which virtually eliminates the unwanted potentials and noise.
In the aforementioned U.S. Pat. No. 3,840,438, it is taught that ion selective electrodes may be preconditioned to the test solution in order to provide a quicker electrical response, and reduce transient errors. Such error elimination, however, does not direct itself to the above-mentioned problems associated with the inert segments within the solution stream. The solutions contemplated for particular use in the pH cell embodiment described hereinafter, are medium to poorly buffered and of low conductivity. It should be understood, however, that the invention is not limited solely to pH cells, but is useful in all ionic measurements featuring inert segmented solution streams.